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ADSORPTION  0?  Ti;\"  BY  PROTEINS  AND 

ITS  RELATION  TO  THE  SOLUTION  OF 

TIN  BY  CANNED  FOODS 

by 
B.C.  G0S3 


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Adsorption  of  Tin  by  Proteins  and 

Its  Relation  to  the  Solution  of 

Tin  by  Canned  Foods 


A  DISSERTATION 

presented  to  the 

Faculty  of  Princeton  University 

IN  Candidacy  for  the  Degree 

of  Doctor  of  Philosophy 

BY 

B.  C.  GOSS 


Accepted  by  the  Department  of  Chemistry,  June,  19 14. 


Adsorption  of  Tin  by  Proteins  and 

Its  Relation  to  the  Solution  of 

Tin  by  Canned  Foods 


B.  C.  GOSS 


189515 


Digitized  by  the  Internet  Archive 

in  2007  with  funding  from 

IVIicrosoft  Corporation 


http://www.archive.org/details/adsorptionoftinbOOgossiala 


ADSORPTION  OF  TIN  BY  PROTEINS  AND  ITS  RELATION 
TO  THE  SOLUTION  OF  TIN  BY  CANNED  FOODS 

The  presence  of  tin  in  foods  which  have  been  packed 
in  tin  cans  has  long  been  known  and  a  great  amount  of 
work  has  been  done  on  this  subject,  especially  since 
1878,  when  Menke  published  an  article  on  "Tin  in 
Canned  Foods. "^  This  work  has,  however,  been  al- 
most entirely  concerned  with  the  mere  presence  of 
tin,  determination  of  total  tin  present  and  with  meth- 
ods for  recovering  it.'*'^*  The  general  procedure  is  to 
destroy  first  the  organic  matter.  This  is  done  by  wet 
or  dry  oxidation  or  a  combination  of  the  two. 

In  the  dry  oxidation,  the  food  is  evaporated  and  the 
dry  mass  charred  and  oxidized  in  a  muffle  furnace,  a 
small  amount  of  potassium  nitrate  or  nitric  acid  as- 
sisting in  the  operation.  The  tin  is  left  in  an  insolu- 
ble form  as  stannic  oxide.  It  is  then  rendered  solu- 
ble by  fusion  with  sodium  carbonate  and  sul- 
fur or  with  caustic  potash,  giving,  respectively, 
sodium  sulfostannate  or  potassium  stannate.  Also 
the  stannic  oxide  may  be  reduced  to  metallic  tin  by 
a  stream  of  hydrogen  gas  at  red  heat  or  fused  with 
potassium  cyanide  and  the  metal  dissolved  in  hydro- 
chloric acid. 

The  moist  incineration  processes  involve  oxidation 
of  organic  matter  by  nitric  acid,  hydrochloric  acid 
and  potassium  chlorate,  sulfuric  acid  and  potassium 
sulfate  or  by  a  mixture  of  nitric  and  sulfuric  acids. 
In  the  latter  two  cases  the  tin  is  left  in  soluble  form,  as 
stannic  sulfate,  without  any  volatile  compounds 
being  formed  which  might  cause  a  loss  of  part  of  the 

>  Chem.  News,  38,  5. 

*  Analyst,  1880,  p.  218. 
'  Chem.  News,  48,  257. 

*  Chem.  Ztg..  24,  263. 

»  Z.  Nahr.  Genussm.,  3,  246. 

•  Chem.  Ztg..  23,  854. 
'  Arch.  Hyg..  46. 

»  Z.  Nahr.  Genussm.,  7,  676. 

•  U.  S.  Dept.  Agr..  Bureau  of  Chemistry,  Bull.  107,  61. 

!•  Report  No.  7,  Local  Government  Board,  Gt.  Britain  (1908). 

"  U.  S.  Dept.  Agr.,  Bureau  of  Chemistry,  Bull.  137. 

"  Ibid.,  67. 

«"  J.  Ind.  Eng.  Chem.,  5  (1913),  3. 

"  8th  Intern.  Congr.  Appl.  Chem.,  18,  35. 


tin.  Having  destroyed  the  organic  matter  and  hav- 
ing the  tin  ixi  solution,  the  amount  may  be  determined 
either  gravimetrically  or  by  one  of  several  volumetric 
methods,  all  of  which  depend  upon  the  conversion  of 
stsLTinous  to  stannic  salts.  We  have  adopted,  for  the 
purpose  of  this  investigation,  the  method  worked  out 
by  H.  A.  Baker,  now  of  the  American  Can  Company,^ 
and  used  with  slight  variations  by  the  Bureau  of 
Chemistry,  American  Can  Company  and  the  National 
Canners'  Association. 

A  new  method  may  be  mentioned  here  which  was 
tried  for  determining  the  tin  in  our  solutions.  We 
have  found  that  the  organic  matter  may  be  easily 
and  quickly  destroyed  by  perchloric  acid  at  its  boil- 
ing point  where  the  approximate  composition*  is 
HCIO4.2H2O,  or,  especially  by  a  mixture  of  perchloric 
and  nitric  acids  from  which  the  nitric  acid  may  then 
be  easily  driven  off.  The  salts  of  perchloric  acid 
are  perfectly  stable,  readily  soluble  and  not  reduced 
by  electrolysis  so  it  was  thought  that  the  tin  might 
be  very  accurately  determined  by  electrolysis  of  this 
perchloric  acid  solution.  We  found  that  by  using 
a  mercury  surface  of  200  cm^.  as  the  cathode,  with 
which  the  tin  is  easily  amalgamated  while  the  over- 
voltage  of  the  hydrogen  is  at  a  maximum,  tin  ions 
could  be  completely  and  quickly  removed  from  large 
volumes  of  dilute  solution.  In  the  removal  of  the 
mercury  by  distillation,  however,  difficulties  were 
encountered,  owing  to  the  tendency  of  the  tin  to  oxi- 
dize and  stick  to  the  walls  of  the  flask.  We  expect  to 
do  more  work  along  this  line. 

Little  or  no  exact  information  has  been  obtained  re- 
garding the  mechanism  of  the  solution  of  the  tin  by 
the  canned  food  nor  the  condition  in  which  it  is  pres- 
ent. Bigelow  and  Bacon'  compared  the  acidity  of  a 
large  number  of  canned  foods  with  the  total  tin  pres- 
ent, and  there  appears  to  be  little  relation  between  the 
two.  For  example,  beets  packed  in  plain  tin  cans 
were  found,  6  months  after  packing,  to  contain  72.8 
mg.  of  tin  per  100  mg.  of  acid,  while  cherries  con- 
tained only  i.s  mg.  of  tin  per  100  of  acid.  J.  P. 
Atkinson  noticed  that  if  tin  salts  were  added  to  meats, 
only  a  third  to  a  half  of  the  tin  could  be  recovered  by 
electrolysis  even  after  an  artificial  gastric  digestion.* 
We  have  noticed  that  in  the  electrolysis  of  a  pulped 

»  8th  Intern.  Cong.  Appl.  Chem.,  18,  35. 

«  J.  Am.  Chem.  Soc,  34  (1912),  1480. 

»  J.  Ind.  Eng.  Chem.,  3  (1911),  832. 

*  J.  P.  Atkinson.  Bureau  of  Health.  New  York  (unpublished). 


food  sample  over  a  mercury  cathode  only  a  part  of 
the  tin  was  deposited,  even  after  a  much  longer  time 
than  is  usually  necessary.  Evidently  the  tin  is  not 
entirely  in  solution.  Some  evidence  on  this  point 
was  found  in  the  experiments  of  linger  and  Bodlander, 
confirmed  by  Buchanan  and  Schryver,  in  which  the 
food  was  roughly  separated  into  liquid  and  solid  por- 
tions by  a  sieve  and  each  analyzed  separately  for 
tin^'^.  The  solid  portion,  of  course,  still  contained 
large  amounts  of  liquid  but  the  results  showed  an  un- 
equal distribution  of  tin  between  the  liquid  and  solid 
portions. 

It  is  obvious  from  this  brief  review  of  the  situation 
that  the  first  question  to  be  settled  is  exactly  the  one 
of  how  much  tin  is  in  true  solution  in  the  various 
kinds  of  canned  foods  as  well  as  the  total  amount  of 
tin  present. 

EXPERIMENTAL 

We  have  succeeded  in  making  a  satisfactory  separa- 
tion of  the  tin  which  is  in  true  solution  from  the  com- 
bined tin  by  means  of  dialysis.  Owing  to  the  ease  with 
which  tin  salts  hydrolyze,  precautions  had  to  be  taken 
to  avoid  hydrolysis  during  the  dialysis.  The  follow- 
ing scheme  was  adopted.  The  bottom  was  cut  off 
from  a  wide,  two-liter  bottle  and  replaced  by  a  film 
of  collodion  which  was  made  by  pouring  out  the  col- 
lodion upon  a  dish  of  mercury  and  before  entirely 
hard,  pressing  it  upon  the  glass.^  This  makes  a  mem- 
brane which  is  very  strong  and  capable  of  being  used 
for  several  determinations  before  requiring  replace- 
ment and,  therefore,  owing  to  ease  of  preparation, 
strength,  and  the  short  time  required  for  dialysis,  it 
was  chosen  in  preference  to  gold-beaters'  skin  and  thin 
parchment  which  were  also  tried.  The  acidity  of  the 
sample  of  food  was  determined  directly  on  removal 
from  the  can  by  titrating  20  cc.  of  the  filtered  juice, 
using  phenolphthalein  as  indicator,  against  N/io 
sodium  hydroxide.  If  the  juice  was  too  darkly  col- 
ored, azolitmin  on  a  spot  plate  was  used.*  Informa- 
tion regarding  the  character  of  the  acid  was  obtained 
in  most  cases  from  the  work  of  Bigelow  and  Dunbar, 
"Acid  Content  of  Fruit  Juices."^  In  most  berries 
the  acidity  is  due  chiefly  to  citric  acid  while  in  the 
stone  fruits,  such  as  cherries,  plums,  peaches,  apples, 

•  Beckurts,  Jahresber.,  46. 

2  Report  No.  7,  Local  Government  Board,  Gt.  Britain  (1908). 

3  Bigelow  and  Gemberling,  Amer.  Chem.  J.,  29  (1907),  1576. 

♦  Bigelow  and  Dunbar.  "Acid  Content  of  Fruits"  (unpublished). 


8 

apricots   and   most  pears,   the  predominating  acid  is 
malic. 

One  liter  of  an  acid  solution  of  the  same  kind  and 
strength  as  that  of  the  liquid  of  the  canned  food  was 
placed  in  a  high  crystallizing  dish  and  the  dialyzer 
suspended  in  this  solution.  A  weighed  sample  of  the 
pulped  fruit  was  placed  inside  and  constantly  stirred 
so  as  to  present  a  fresh  surface  to  the  membrane.  A 
battery  of  8  dialyzers  was  stirred  from  a  central  re- 
volving shaft.  It  was  found  that  in  about  48  hours 
the  equilibrium  was  established,  although  in  some  cases 
a  longer  time  was  allowed,  and  after  this  interval 
the  dialyzer  was  raised  and  the  volume  of  the  contents 
inside  and  out  measured.  The  large  volume  of  the 
solution  outside  the  membrane  was  evaporated  and 
transferred  to  a  Kjeldahl  flask  and  the  residue  of  pulp 
inside  to  another:  100  cc.  of  concentrated  nitric  acid 
were  added  to  each  and  the  mixtures  let  stand.  If 
the  food  sample  contained  much  sugar,  rapid  oxida- 
tion began  almost  at  once  and  the  flasks  were  left 
until  brown  fumes  ceased  to  come  off  when  50  cc. 
of  concentrated  sulfuric  acid  were  added  and  heat 
applied,  thus  avoiding  too  violent  action.  After 
heating  until  dense  fumes  of  sulfuric  acid  appeared, 
the  flasks  ^ere  cooled  and  in  case  the  solution  was 
not  colorless,  small  portions  of  nitric  acid  were 
added  successively  and  heating  repeated.  The  finally 
clear  solution,  from  which  all  nitric  acid  had  been  ex- 
pelled, was  cooled,  diluted  with  water  and  the  acid 
neutralized  with  concentrated  ammonia,  testing  with 
litmus  paper  and  then  the  solution  was  acidified 
slightly  with  hydrochloric  acid,  heated  to  boiling 
and  hydrogen  sulfide  passed  in  until  the  tin  was  all 
precipitated.  The  precipitates  were  allowed  to  set- 
tle and  filtered  in  pairs,  by  suction,  through  asbestos, 
using  false  bottom  Gooch  crucibles.  The  precipi- 
tates were  washed  with  hot  water  which  had  been 
saturated  with  hydrogen  sulfide.  The  tin  sulfide 
was  dissolved  in  Erlenmeyer  flasks  by  boiling  with 
concentrated  hydrochloric  acid,  to  which  successive 
small  portions  of  potassium  chlorate  were  added  and 
the  chlorine  expelled  at  the  end  of  the  addition  of  a 
gram  of  aluminum  foil.  The  flasks,  four  at  a  time, 
were  placed  upon  a  hot  plate  and  attached  to  a  car- 
bon dioxide  generator.  After  all  the  air  had  been 
displaced  by  carbon  dioxide,  the  tin  was  reduced  to 
the  stannous  condition  by  the  addition  of  about  2 
g.  of  aluminum  foil.     The  solutions  were  boiled  for  a 


few  minutes  after  the  aluminum  disappeared  and  then 
cooled  in  ice-water,  still  in  an  atmosphere  of  carbon 
dioxide,  removed  one  at  a  time,  tubes  and  stoppers 
washed  down  with  air-free  water  and  titrated  with 
N/ioo  iodine  solution,  using  starch  as  indicator. 
Each  time  a  series  of  titrations  was  made  the  iodine 
was  standardized  against  a  tin  solution,  i  cc.  of  which 
contained  i  mg.  of  tin.  Knowing  the  amount  of  tin 
in  the  solution  outside  the  membrane,  from  the  rela- 
tive volumes  of  the  acid  solution  and  of  the  food  pulp, 
the  total  amount  of  tin  which  was  in  true  solution 
was  calculated  and,  by  .difference,  the  tin  which  was 
in  an  insoluble  form.  The  determinations  were  car- 
ried out  in  pairs  and  the  average  of  results  given.  In 
some  cases,  as  that  of  rhubarb,  the  agreement  was 
exceptionally  close,  the  pair  yielding,  respectively, 
8.9  and  9.17  mg.  of  insoluble  tin  in  a  75-g.  sample. 
Where  a  large  percentage  of  the  tin  was  in  an  insolu- 
ble form,  however,  the  agreement  was  not  so  close, 
due  partly  at  least  to  the  impossibility  of  getting  two 
samples  having  just  the  same  proportions  of  liquid 
and  solid,  and  therefore  in  which  the  insoluble  tin 
compound  was  equally  distributed.  A  determina- 
tion of  the  total  tin  in  the  sample  of  food  used  was  also 
made  in  the  usual  way. 

It  will  be  noticed  that  in  Table  I  the  foods  examined 
are  arranged  in  the  order  of  their  increasing  acidity 
as  shown  in  column  9.  It  is  obvious  that  neither 
the  total  tin  nor  the  tin  which  is  in  solution  are  directly 
proportional  to  the  acidity  and  it  is  evident  that  the 
amount  of  tin  which  is  removed  from  the  can  is  de- 
pendent also  upon  other  factors.  In  calculating  the 
amount  of  tin  in  solution  in  the  pulp,  from  the  concen- 
tration of  tin  outside  the  membrane  after  dialysis 
and  the  volume  of  the  pulp  it  was  assumed  that  the 
tin  in  actual  solution  was  free  to  diffuse  throughout 
the  whole  volume  of  the  pulp;  that  is,  that  the  space 
occupied  by  the  solid  particles  of  the  food  did  not  les- 
sen the  volume  over  which  the  soluble  tin  could  dis- 
tribute itself.  In  order  to  determine  the  maximum 
possible  error  which  might  arise  from  this  source,  the 
volume  of  the  solids  was  determined  in  the  case  of  two 
of  the  foods  which  contained  the  highest  percentage 
of  tin  in  solution,  for  it  would  be  in  such  foods  that 
the  error  must  be  greatest.  A  weighed  sample  of 
rhubarb,  similar  to  the  one  used  in  the  dialysis,  was 
sucked  dry  of  liquid  in  a  Buchner  funnel  and  the  solid 
residue   immersed   in   a   measured   amount   of    water, 


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noting  the  increase  in  volume.  This  was  found  to 
be  only  0.3  cc.  so  that  the  volume  of  the  pulp  inside 
the  membrane  through  which  tin  could  diffuse  was 
189.7  instead  of  190.0  cc.  Calculating  the  amount 
of  tin  in  solution,  on  this  basis,  in  the  pulp  we  found 
26.22  instead  of  26.26  mg.,  a  difference  of  0.04  mg., 
which  is  negligible.  Next  to  rhubarb,  beets  contain 
the  highest  percentage  of  tin  in  solution,  and  the 
difference  found  in  the  amount  of  tin  in  solution  in 
the  pulp,  when  the  volume  of  the  solid  in  the  pulp 
was  taken  into  consideration,  was  0.02  mg.  out  of  a 
total  of  9.63  mg.  From  these  results  we  have  con- 
cluded that  the  volume  actually  occupied  by  the  solid 
in  the  pulp  may  be  neglected  and  the  tin  in  the  solu- 
tion in  the  pulp  calculated  as  if  it  were  equally  dis- 
tributed over  the  total  volume  inside  the  membrane. 

It  will  be  observed  from  Table  I  that  rhubarb,  which 
was  the  first  of  the  fruits  examined,  showed  a  small 
percentage  of  tin  in  an  insoluble  form,  while  pumpkin, 
squash,  string  beans  and  other  foods  high  in  proteins, 
contained  a  large  amount  of  tin  which  was  no  longer 
in  solution.  We  expected  then  that  in  the  case  of 
the  berries,  which  are  rather  strongly  acid  and  con- 
tain almost  no  protein  matter,  the  greater  part  of  the 
tin  would  be  found  in  solution  as  was  determined  for 
rhubarb.  When  we  came  to  examine  raspberries, 
however,  we  were  much  surprised  to  find  so  high  a  per- 
centage of  the  tin,  about  81  per  cent,  in  an  insoluble 
form.  The  same  was  true  in  varying  degrees  for  other 
similar  fruits,  strawberries,  gooseberries,  currants, 
cherries,  etc.  Since  there  appeared  to  be  some  rela- 
tion between  the  amount  of  protein  matter  and  the 
part  of  the  tin  which  was  insoluble,  and  since  the  only 
proteins  in  berries  are  in  the  nuclei  of  the  seeds, 
some  of  these  seeds  were  analyzed  for  tin. 

The  whole  raspberries,  containing  180  mg.  of  tin 
per  kg.,  were  pulped  and  pressed  through  cloth,  the 
seeds  being  removed  from  the  solid  residue  by  washing 
and  decantation  in  a  large  crystallizing  dish.  In  this 
way  perfectly  clean  seeds,  free  from  pulp,  were  ob- 
tained. These  were  then  washed  with  boiling  water, 
dried  in  air  and  the  tin  determined  in  the  usual  way. 
The  tin  in  the  seeds  ran  805  mg.  per  kg.  In  other 
words,  most  of  the  tin  which  is  in  an  insoluble  form 
was  found  in  the  seeds.  In  strawberries,  this  reten- 
tion of  tin  is  even  more  marked.  The  seeds  of  straw- 
berries were  found  to  contain,  roughly,  six  times  as 
much  tin,  weight  for  weight,  as  the  whole  fruit — the 


Total 

Per 

Tin 

cent 

Seeds 

Acidity 

Mg. 

Insoluble 

Mg. 

Per  cent 

per  Kg. 

Tin 

per  Kg. 

Malic  0.70 

222.0 

67.2 

448.0 

Malic  0.51 

90.0 

83.3 

321.0 

Citric  0.71 

180.0 

81.6 

805.6 

Citric  0.70 

416.0 

55.5 

2630.0 

Citric  0.39 

70.0 

37.1 

106.5 

seeds  gave  2630  mg.  per  kg.  as  compared  to  416  for 
the  whole  fruit.  The  fact  that  the  larger  part  of  the 
tin  in  the  berries  mentioned  is  combined  and  insoluble 
in  the  seeds  is  of  fundamental  importance  in  deter- 
mining the  physiological  action  of  tin  in  canned  foods, 
for  the  seeds,  and  with  them  the  adsorbed  tin  will 
be  eliminated,  to  a  large  extent  at  least,  directly  in  the 
feces. 

On  the  basis  of  these  experiments,  it  would  appear 
that  the  amount  of  soluble  tin  salts,  rather  than  the 
total  tin  present  in  a  can  of  food,  should  be  limited, 
since  it  is  the  part  of  the  tin  adsorbed  which  deter- 
mines the  physiological  action.  A  few  typical  de- 
terminations on  fruit  seeds  appear  in  Table  II. 

Table  II — Adsorption  op  Tin  by  Seeds  op  Fruits 


Age 
No.        Food  Sample  Yrs. 

1  Red  cherries 5 

2  Black  cherries  (enamel) .      5 

3  Red  raspberries  (enamel)     5 

4  Strawberries 5 

5  Tomatoes 1 


We  have  already  mentioned  the  fact  that  beets 
and  rhubarb,  the  first  of  the  foods  examined,  contain 
almost  no  protein  and  that  in  these  foods  we  found 
large  amounts  of  tin  in  solution.  The  foods  high  in 
proteins,  such  as  string  beans,  squash  and  pumpkin, 
next  to  be  investigated,  showed  a  high  percentage 
of  tin  in  an  insoluble  form.  In  berries,  we  found  that 
the  greater  part  of  the  tin  was  concentrated,  with  the 
protein,  in  the  seeds.  It  seemed  from  these  results 
that  there  was  some  connection  between  the  amount 
of  protein  in  the  food  and  the  percentage  of  tin  in 
solution  as  well  as  the  total  amount  of  tin  removed 
from  the  inner  surface  of  the  can. 

In  order  to  get  further  evidence  on  the  part  played 
by  proteins  in  determining  the  action  of  canned  foods 
on  the  tin  can,  the  following  experiments  were  car- 
ried out:  Coagulated  globulins,  prepared  by  heating 
the  lo  per  cent  sodium  chloride  extract  from  dried, 
pulverized  pea  beans  (soup  beans),  were  washed  and 
suspended  in  water  in  contact  with  tin  plate  of  392  sq.cm. 
surface  and  the  tubes  sealed.  After  two  weeks,  an 
average  of  0.6  mg.  of  tin  was  found  combined  with 
the  protein.  Also,  it  was  found  that  proteins,  sealed 
with  dilute  acid  solutions  in  contact  with  tin,  greatly 
increase  the  amounts  of  tin  going  into  solution  (Table 
III).     In  these  experiments  a  coil  of  tin  plate,  having 


13 

Table  III — Influence  of  Agar  Jell.  Proteins,  Etc.,  on  Solution  of 
Tin  by  Citric  Acid 

Time  Mg.  Tin 

(Mo.)  Dissolved 

Citric  Acid  (5%) 2  17.9 

7  20.5 

Citric  Acid  (5%)  +  Agar  Jell 2  18.3 

4  25.0 

7  25.3 

Citric  Acid  (5%)  +  Proteins 4  32.7 

4  34.6 

Citric  Acid  (2%) 7  11.7 

Citric  Acid  (2%)  +  Agar  Jell 7  16.9 

Citric  Acid  (2%)  +  Crushed  Peas 7  35.5 

Citric  Acid  (2%)  +  String  Beans 7  32.5 

a  surface  of  392  sq.  cm.,  was  sealed  in  contact  with  a 
constant  volume  of  citric  acid  solution  (100  cc),  a 
part  of  the  tubes  containing  citric  acid  alone,  the 
others  having  coagulated  proteins,  agar  jell,  etc., 
added.     All  were  kept  at  the  same  temperature. 

The  simple  first  reaction  of  the  acid  in  the  can  of 
food  is  complicated  by  the  presence  of  large  amounts 
of  colloidal  proteins  which  undoubtedly  affect  the 
solution  of  tin.  Albumins,  globulins  and  other  pro- 
teins are  negative  colloids  and  are  precipitated  by  an 
ion  of  opposite  charge.  This  is  especially  true  of  the 
heavy  metal  ions,  of  which  tin  is  an  example,  and  this 
precipitation  is  irreversible.  It  is  known  that  when 
a  sol  is  thus  precipitated,  the  precipitating  ion  is  firmly 
adsorbed  and  carried  down  with  it.  Linder  and  Picton 
first  observed  this  in  the  case  of  arsenious  sulfide 
sol  and  barium  chloride.^  In  such  cases  the  solu- 
tion remaining  is  found  to  be  strongly  acid  and  in  the 
same  degree  in  which  the  precipitate  contains  the 
metal  ion.  These  precipitates  hold  the  metal  ion 
very  firmly  and  no  amount  of  washing  will  remove  it. 
In  some  respects  they  appear  to  be  true  chemical 
compounds,  but  the  composition  is  too  variable  to 
admit  of  this  view.  For  example,  precipitates  formed 
by  the  action  of  copper  salts  on  albumins  contain 
all  the  way  from  1.4  to  20  per  cent  of   copper  oxide.* 

These  facts  observed  for  other  heavy  metals  agree 
closely  with  the  facts  observed  in  the  combination  of 
tin  with  food  materials.  After  a  small  amount  of 
tin  has  been  dissolved  from  the  surface  of  the  can, 
adsorption  and  precipitation  take  place.  When  the 
tin  ion  is  removed  from  solution  by  the  proteins,  the 
acid  ion  is  liberated  and  more  tin  dissolved.  In  this 
way  the  tin  would  be  constantly  removed  from  solu- 
tion and  a  small  concentration  of  acid  could  ultimately 
dissolve  a  very  large  amount  of  tin.  If  the  cell  walls 
surrounding    the    colloidal    proteins    were    unbroken, 

«  Chem.  Soc.  Jour.,  67  (1895).  63. 

•  W.  W.  Taylor.  "Chemistry  of  Colloids,"  p.  118. 


the  proteins  could  not  diffuse  out  into  the  solution, 
but  the  tin  could  enter  and  adsorption  take  place. 
Since  practically  all  of  the  action  of  the  food  on  the 
container  takes  place  after  processing,  which  involves 
heating  to  a  rather  high  temperature,  most  of  our 
proteins  have  been  coagulated,  but  this  seems  to  have 
little  or  no  effect  on  the  removal  of  tin  from  solution, 
and  coagulated  proteins  were  found  to  take  up  large 
quantities  of  tin.  Beans  were  pulverized  with  sand, 
extracted  first  with  water,  obtaining  a  solution  of 
proteoses  and  albumins,  and  then  with  a  lo  per  cent 
sodium  chloride  solution  which  removed  large  quan- 
tities of  globulins.  These  solutions  and  egg  albumen 
were  used  for  the  following  tests:  Small  volumes  of 
each  of  the  above  solutions  were  added  to  an  excess 
of  2.5  and  5  per  cent  stannic  chloride  and  stannic 
ammonium  chloride  solutions  and  the  precipitate 
which  was  formed  filtered,  washed  several  times  in 
boiling  water,  dried  at  110°  C.  and  the  percentage  of 
tin  in  a  weighed  sample  determined  gravimetrically. 
Parts  of  the  same  protein  solutions  were  coagulated 
by  heat  and  the  coagulated  proteins  suspended  in  the 
same  tin  solutions  for  two  days.  The  results  show  a 
varying  percentage  of  tin  which,  however,  is  uni- 
formly high.  It  was  also  noticed  that  if  the  precipi- 
tate was  filtered  and  washed  and  one  part  of  it  dried 
and  the  percentage  of  tin  determined,  while  the  other 
part  was  put  back  in  the  solution  and  let  stand,  it 
continued  to  adsorb  more  tin.  For  example,  40  cc. 
of  dilute  globulin  solution  in  10  per  cent  NaCl  were 
added  to  400  cc.  of  5  per  cent  stannic  ammonium 
chloride;  a  white  precipitate  formed  which  was  warmed 
to  complete  the  coagulation  and  let  stand  for  a  day, 
then  filtered,  washed  and  dried;  0.409  g.  gave  on  anal- 
ysis 0.227  g-  of  till  or  about  55.5  per  cent.  A  part 
of. the  same  precipitate  was  left  in  contact  with  the 
solution  for  a  week  before  filtering,  when  0.777  g- 
showed  0.441  g.  of  tin  or  60.6  per  cent.  Using  a  5 
per  cent  stannic  chloride  solution  the  percentage  of 
tin  in  one  case  ran  to  69.2.  As  might  be  expected, 
the  percentage  of  tin  increases  with  the  concentra- 
tion   of   the   solution.     This   is   shown   in    Table    IV. 

Tablb  IV — Adsorption  op  Tin  by  Coagulated  Proteins 
Original 

concentration                Final  Per  cent 

Gm.  of  Tin  concentration  Tin 

(as  SnCh)  Gm.  of  Tin  in  Dried 

per  cc.                     per  cc.  Protein 

A 0.000310  0.0000019  4.92 

B 0.000517  0.0000465  9.90 

C 0.001610  0.0009730  20.50 

D 0.004680  0.0041600  35.60 


15 

Stannic  chloride  solutions,  of  varying  concentrations, 
were  made  up  and  concentrated  hydrochloric  acid 
added  to  each  to  prevent  hydrolysis.  A  constant 
weight  of  coagulated  protein^  1.5  g.,  was  suspended 
in  350  cc.  of  each  solution  and  left  for  a  week,  after 
which  a  portion  of  the  clear  liquid  was  withdrawn 
with  a  pipette  and  analyzed  for  tin,  and  the  protein 
was  filtered  off,  washed  with  several  portions  of  boil- 
ing water,  dried  at  110°  C,  and  the  percentage  of  tin 
determined.  It  will  be  noticed  that  in  each  case 
tin  was  left  in  solution. 

Experiments  were  made  with  the  insoluble  tin  com- 
pound from  several  of  the  canned  foods  which,  al- 
though but  slightly  acid,  contained  large  amounts 
of  tin  and  it  was  found  that  here,  too,  the  tin  is  very 
firmly  bound.  Squash  is  a  good  example  of  this. 
Samples  of  squash,  which  had  been  packed  in  tin  cans 
and  contained  300  mg.  of  tin  per  kg.,  were  boiled  for 
about  5  hours  with  water  and  the  three  protein  sol- 
vents, 10  per  cent  NaCl  solution,  70  per  cent  alcohol 
and  2  per  cent  HCl,  and  filtered  through  hardened 
filters.  In  the  first  three  cases — water,  alcohol  and 
salt  solutions — only  32.5,  32.8  and  35.8  per  cent, 
respectively,  of  the  tin  was  found  in  solution.  The 
hydrochloric  acid  seems  to  break  up  the  tin  com- 
pound slowly  on  boiling  and  after  5  hours  25.4  per 
cent  of  the  tin  was  still  found  combined  with  the  solid 
residue.  The  question  as  to  whether  the  tin  which 
is  adsorbed  by  these  proteins  passes  through  the  pro- 
cesses of  digestion  without  being  absorbed  is  of  first 
importance.  We  have  mentioned  this  point  in  re- 
gard to  the  tin  which  was  found  combined  in  the  seeds 
of  berries  and  in  addition  have  performed  the  follow- 
ing experiments  to  obtain  further  information. 

Artificial  gastric  digestions  were  carried  out  upon 
the  solid  residue  obtained  by  boiling  canned  squash, 
which  contained  300  mg.  of  tin  per  kg.  with  water 
and  filtering.  This  solid  residue  contained  about  67 
per  cent  of  the  total  tin  in  the  squash  sample.  The 
gastric  juice,  pepsin  in  0.35  per  cent  HCl,  was  added 
to  the  squash  and  the  mixture  kept  in  a  thermostat 
at  36°  C.  for  24  hrs.,  after  which  it  was  transferred 
to  a  dialyzer  and  the  tin  in  solution  determined  in  the 
usual  way.  Less  than  10  per  cent  of  the  tin  was  found 
in  solution.  Both  gastric  and  tryptic  digestions  were 
kindly  carried  out  for  us  by  Dr.  E.  N.  Harvey,  of  the 
Biology  Department,  on  the  tin  protein  complex, 
prepared  by  allowing  the  freshly   coagulated  protein 


i6 

to  stand  in  contact  with  tin  solutions,  after  it  had  been 
allowed  to  dry,  and  in  each  case  only  a  trace  of  tin  was 
found  in  solution.  It  appears  from  the  above  results 
that  the  tin  protein  combination  which  is  formed  is 
very  stable,  and  in  most  of  the  foods  containing  the 
larger  amounts  of  tin,  the  greater  part  is  in  an  insolu- 
ble form.  The  possibility  suggests  itself  that  the 
part  of  the  tin  which  is  so  firmly  adsorbed  will  be 
eliminated  directly  in  the  actual  digestive  processes 
and  not  figure  in  the  physiological  action  as  deter- 
mined for  soluble  tin  salts. 

The  work  of  J.  P.  Atkinson  on  the  electrolysis  of 
metallic  salt  solutions  to  which  chipped  beef  had  been 
added  is  of  interest  in  this  connection.  A  known 
amount  of  the  metal  in  the  form  of  a  soluble  salt  was 
added  to  finely  divided  beef  and  then  submitted  to 
artificial  gastric  digestion  for  24  hrs.  at  37°  after 
which  the  solution  was  electrolyzed  for  45  to  50  hrs. 
A  few  typical  results  follow: 

Added  Recovered  Difference  Per  cent 

Mbtai,  Gram  Gram              Gram  Recovered 

Mercury 0.0500  0.0121  —0.0379            24.1 

Mercury 0.0500  0.0217  —0.0283            43.4 

Tin 0.0330  0.0051  —0.0279            15.5 

Tin 0.0330  0.0063  —0.0267            19.1 

Zinc 0.0500  0.0561  +0.0061  100.0 

Nickel 0.0492  0.0497  +0.0005  100. 0 

Iron 0.0500  0.0497  —0.0003            99.7 

It  appears  that  the  metals  of  relatively  low  toxicity 
are  least  firmly  bcnind  and  he  suggests  that  this  may 
offer  an  explanation  of  the  relative  toxicity  of  metals, 
in  that  they  interfere  with  the  metabolism  of  the  cell. 
Iron,  being  so  easily  separated,  adds  to  this  view.  It 
was  found  that  the  toxicity  of  mercury  as  the  bichloride 
was  greatly  diminished  by  adding  it  to  chopped  meat 
and  submitting  it  to  an  artificial  gastric  digestion. 
One  mg.  of  mercury  as  bichloride  will  kill  a  2So-g. 
guinea  pig  in  4  hrs.  if  injected  subcutaneously,  toxic 
symptoms  beginning  in  a  few  minutes.  The  same 
quantity  of  mercury,  after  combining  it  with  tissue 
as  described  above,  produced  no  toxic  symptoms  and 
death  did  not  follow  until  the  fifth  day.  Rabbits 
also  were  injected  without  apparent  harmful  effects. 

RESUME    AND    CONCLUSIONS 

It  has  been  shown  that  the  solution  of  tin  by  canned 
foods  is  neither  dependent  upon,  nor  proportional  to, 
the  acidity  alone  and,  also,  that  in  the  foods  of  rela- 
tively slight  acidity  which  dissolve  large  amounts  of 
tin,  the  greater  part  of  the  tin  is  in  the  form  of  an  in- 
soluble and  stable  complex.     The  explanation  which 


«7 

agrees  most  closely  with  the  observed  facts  is  that 
we  are  dealing  here  with  adsorption  phenomena;  that 
the  tin,  after  being  dissolved  from  the  lining  of  the 
can,  is  being  constantly  removed  from  solution  by 
the  proteins,  carbohydrates  and  other  highly  porous 
solid  phases  in  contact  with  the  solution.  Whether 
we  regard  this  as  an  adsorption  of  tin  ions,  or  whether 
we  consider  the  tin  salt  to  be  first  hydrolyzed  and  the 
resulting  stannous  hydroxide  adsorbed,  in  either  case 
the  acid  would  be  regenerated  and  able  to  attack  more 
tin.  The  former  explanation  seems  to  be  the  more 
probable;  i.  e.,  the  tin  ions  are  adsorbed,  since  tin 
is  taken  up  equally  well  by  proteins  even  from  con- 
centrated acid  solution.  It  will  be  seen  from  the 
above  results  that  while  in  several  respects  the  ob- 
served phenomena  appear  to  be  true  adsorptions, 
in  one  important  respect  they  differ.  While  a  true 
adsorption  is  an  equilibrium  and  can  be  approached 
from  either  side,  being  reversible,  this  removal  of  tin 
is  not  a  reversible  action,  for  if  t^he  tin  protein  complex 
is  transferred  to  an  aqueous  solution  containing  no 
tin,  it  does  not  lose  tin  to  the  liquid  phase.  A  num- 
ber of  cases  similar  to  this  are  known  and  have  been 
called  by  W.  W.  Taylor,  "Pseudo-adsorptions."* 
The  removal  of  heavy  metal  salts  from  solution  by 
charcoal  is  an  example  of  this  type  of  action;  the  first 
stage  may  be  an  adsorption,  since  the  salts  of  heavy 
metals  are  strongly  adsorbable,  but  a  secondary  re- 
action must  have  taken  place  and  the  final  state  can- 
not be  put  down  to  adsorption  alone. 

The  author  wishes  to  express  his  appreciation  and 
thanks  for  the  very  valuable  assistance  and  advice 
given  by  Dr.  G.  A.  Hulett  in  connection  with  this 
work. 

Laboratory  of  Puysical  Chemistry,  Princeton,  N.  J. 
AND  Bureau  of  Chemistry,  Washington.  D    C. 


»  W.  W.  Taylor.  "Chemistry  of  Colloids."  p.  252. 


189515 


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